Method and device to quantify active carrier profiles in ultra-shallow semiconductor structures

Abstract

A method and device for determining, in a non-destructive way, at least the active carrier profile from an unknown semiconductor substrate are disclosed. In one aspect, the method comprises generating 2 m independent measurement values from the m reflected signals and correlating these 2 m measurement values with 2 m independent carrier profile values. The method further comprises generating additional 2 m measurement values to allow determining the active carrier profile and a second parameter profile by correlating the 4 m measurement values with the 4 m profile values. The method further comprises generating a total of 2 m[n.k] measurement values to allow determining [n.k] independent material parameter depth profiles, each material parameter profile having m points.

1. A method of using optical measurement to determine at least an active carrier profile of a semiconductor substrate, the profile being expressed as a set of concentration values C with corresponding depths Z, the method comprising:
 generating m measurement points, each measurement point comprising two independent measurement signals; and 
 correlating these m measurement points with m active carrier profile points, each active carrier profile point comprising an active carrier concentration C and the corresponding depth Z, with m being an integer value. 
 
     
     
       2. The method of  claim 1 , wherein generating m measurement points comprises:
 providing a pump laser; 
 providing a probe laser; 
 focusing the pump laser and the probe laser on the semiconductor substrate, the pump laser generating in an area of the semiconductor substrate contacted by the pump laser a number of excess charge carriers, having a depth profile, the generated excess charge carriers reflecting the beam of the probe laser; and 
 detecting two independent predetermined characteristics of the reflected probe laser beam. 
 
     
     
       3. The method of  claim 2 , wherein
 the pump laser is selected to create excess carrier plasma waves, and 
 the two independent signals are the amplitude and the phase of the reflected probe laser. 
 
     
     
       4. The method of  claim 2 , wherein generating m measurement points comprises:
 either applying m different values of the power of the probe laser; or 
 applying m different offsets between the pump laser and the probe laser; or 
 detecting two independent predetermined characteristics of the reflected probe laser beam during m different time periods. 
 
     
     
       5. The method of  claim 1 , wherein correlating the m measurement points with the m profile points comprises:
 selecting values for the active concentration and corresponding depth for each of the m profile points; 
 simulating the excess carrier concentration using these selected values to determine the complex refraction index profile of the substrate; 
 determining m values for each of the two independent measurement signals using the simulated refraction index profile; 
 determining the difference between the m determined values with the m measured values for each of the two independent measurement signal; and 
 repeating the above steps until an unique solution for the active carrier profile is obtained. 
 
     
     
       6. The method of  claim 2 , further comprising determining at least another material parameter profile of the semiconductor substrate, the method comprising:
 generating m additional measurement points, each additional measurement point comprising two independent measurement signals; and 
 simultaneously correlating these 2 m measurement points with m active carrier profile points, each active carrier profile point comprising an active carrier concentration C and the corresponding depth Z, and with m material parameter profile points, each material parameter profile point comprising an concentration D and the corresponding depth W with m being an integer value. 
 
     
     
       7. The method of  claim 5 , wherein generating m additional measurement point comprises:
 applying m different values of the power of the probe laser and for each of the m power values varying the offset (d) between the pump laser and the probe laser. 
 
     
     
       8. The method of  claim 5 , wherein generating m additional measurement point comprises:
 applying m different values of the power of the probe laser and for each of the m power values varying the time period t meas  for detecting the two independent predetermined characteristics of the reflected probe laser beam; or 
 applying m different values of the offset (d) between the pump laser and the probe laser and for each of the m offset values varying the time period t meas  for detecting the two independent predetermined characteristics of the reflected probe laser beam. 
 
     
     
       9. The method of  claim 5 , wherein correlating the 2 m measurement points with the 2 m profile points comprises:
 selecting values for the active concentration and corresponding depth for each of the m active carrier profile points; 
 selecting values for the material parameter concentration and corresponding depth for each of the m material parameter profile points; 
 simulating the excess carrier concentration using these selected values to determine the complex refraction index profile of the substrate; 
 determining 2 m values for each of the two independent measurement signals using the simulated refraction index profile; 
 determining the difference between the 2 m determined values with the 2 m measured values for each of the two independent measurement signals; and 
 repeating the above steps until an unique solution for the active carrier profile and for the second parameter profile is obtained. 
 
     
     
       10. The method of  claim 1 , further comprising determining multiple material parameter profiles of the semiconductor substrate, the method comprising:
 generating ([n.k]−1).m additional measurement points, each additional measurement point comprising two independent measurement signals; and 
 simultaneously correlating these [n.k].m measurement points with m active carrier profile points, each active carrier profile point comprising an active carrier concentration C and the corresponding depth Z, and with ([n.k]−1).m material parameter profile points, each material parameter profile point comprising an concentration D and the corresponding depth W with n, k, m being integer values. 
 
     
     
       11. The method of  claim 10 , wherein generating [n.k].m measurement points comprises:
 applying m different values of the power of the probe laser; 
 for each of the m power values, applying n different values of the offset (d) between the pump laser and the probe laser; and 
 for each of the n offset values, applying k different values for the time period t meas  for detecting the two independent predetermined characteristics of the reflected probe laser beam. 
 
     
     
       12. The method of  claim 10 , wherein correlating the [n.k].m measurement points with the [n.k].m profile points comprises:
 selecting values for the active concentration and corresponding depth for each of the m active carrier profile points; 
 selecting values for the material parameter concentration and corresponding depth for each of the ([n.k]−1) m material parameter profile points; 
 simulating the excess carrier concentration using these selected values to determine the complex refraction index profile of the substrate; 
 determining [n.k].m values for each of the two independent measurement signals using the simulated refraction index profile; 
 determining the difference between the [n.k].m determined values with the [n.k].m measured values for each of the two independent measurement signals; and 
 repeating the above steps until an unique solution for the active carrier profile and for the second parameter profile is obtained. 
 
     
     
       13. A computer program product for executing the method of  claim 1  when being executed on a computer device. 
     
     
       14. A machine-readable medium storing a computer program configured to perform the method of  claim 1 . 
     
     
       15. An apparatus for determining at least an active carrier profile of a semiconductor substrate comprising:
 an illumination device comprising:
 a pump laser configured to create excess carriers in the substrate; and 
 a probe laser configured to impinge a laser beam, at least partially reflected by the excess carriers, on the semiconductor substrate, thus generating a reflection signal; 
 
 a measuring module configured to measure the reflection signal; 
 a scanning module configured to scan the active carrier profile when measuring the reflection signal; 
 a storage module configured to store at least m measured reflection signals, each reflection signal comprising two independent signals; and 
 a correlating module configured to correlate the at least m measured reflection signals with m profile points, each profile point comprising an active carrier concentration C and the corresponding depth Z, with m being an integer value. 
 
     
     
       16. The apparatus of  claim 15 , wherein the scanning module further comprises a power varying module configured to vary the power of probe laser. 
     
     
       17. The apparatus of  claim 15 , wherein the scanning module further comprises an offset varying module configured to vary the offset d between the pump laser and the probe laser. 
     
     
       18. The apparatus of  claim 15 , wherein the scanning module comprises a time varying module configured to vary the time during which each reflection signal is measured. 
     
     
       19. The apparatus of  claim 15 , wherein the storage module is adapted for storing 2 m measured reflection signals and the correlating module is adapted for correlating 2 m measured reflection signals with m active carrier profile points and with m second parameter profile points, each profile point comprising a concentration and the corresponding depth, with m being an integer value. 
     
     
       20. The apparatus of  claim 19 , wherein the scanning module comprises two or more of the following:
 a power varying module configured to vary the power of probe laser; 
 an offset varying module configured to vary the offset d between the pump laser for creating excess carriers and the probe laser; and 
 a time varying module configured to vary the time during which each reflection signal is measured. 
 
     
     
       21. The apparatus of  claim 15 , wherein
 the storage module is adapted for storing [n.k].m measured reflection signals; and 
 the correlation module is adapted for correlating [n.k].m measured reflection signals with m active carrier profile points and with ([n.k]−1)m material parameter profile points, each profile point comprising a concentration and the corresponding depth, with m, n, k being integer values. 
 
     
     
       22. The apparatus of  claim 21 , wherein the scanning module comprises:
 a power varying module configured to vary the power of probe laser; 
 an offset varying module configured to vary the offset d between the pump laser and the probe laser; and 
 a time varying module configured to vary the time during which each reflection signal is measured. 
 
     
     
       23. A system for using optical measurement to determine at least an active carrier profile of a semiconductor substrate, the profile being expressed as a set of concentration values C with corresponding depths Z, the system comprising:
 means for generating m measurement points, each measurement point comprising two independent measurement signals; and 
 means for correlating these m measurement points with m active carrier profile points, each active carrier profile point comprising an active carrier concentration C and the corresponding depth Z, with m being an integer value. 
 
     
     
       24. An apparatus for determining at least an active carrier profile of a semiconductor substrate comprising:
 an illumination device comprising:
 means for creating excess carriers; and 
 means for impinging a probe laser beam, at least partially reflected by the excess carriers, on the semiconductor substrate, thus generating a reflection signal; 
 
 means for measuring the reflection signal; 
 means for scanning the active carrier profile when measuring the reflection signal, 
 means for storing at least m measured reflection signals, each reflection signal comprising two independent signals; and 
 means for correlating the at least m measured reflection signals with m profile points, each profile point comprising an active carrier concentration C and the corresponding depth Z, with m being an integer value.